2000 Vauxhall Astra wont go into gear

Hi. My car sometimes wont go into gear, usually first thing in the morning, its ok at first but usually after just a few minutess when at the first junction it doesnt go into any gear, then after a short while( minutes). Its ok. I have had the same problem when I finish work and again after driving a short distance it does the same again. Any ideas please. Thanks